Abu Dawud Book 5, Hadith Number 2155.

Narated By Abdullah ibn Amr ibn al-‘As : The Prophet (PBUH) said: If one of you marries a woman or buys a slave, he should say: “O Allah, I ask Thee for the good in her, and in the disposition Thou hast given her; I take refuge in Thee from the evil in her, and in the disposition Thou hast given her.” When he buys a camel, he should take hold of the top of its hump and say the same kind of thing.